1. Field of the Invention
The present invention relates to a method for manufacturing vertical GaN light emitting diodes, and more particularly to a method for manufacturing vertical GaN light emitting diodes, from which an insulating sapphire substrate with low thermal conductivity is removed and in which a conductive substrate such as a silicon substrate is installed so as to have improved luminance and reliability.
2. Description of the Related Art
Generally, light emitting diodes (LEDs) are semiconductor elements, which emit light based on the recoupling of electrons and holes, and are widely used as various types of light sources in optical communication and electronic equipment. GaN serves as a compound for manufacturing blue-light emitting diodes.
Frequency (or wavelength) of light emitted from the light emitting diode is functionally related to a band gap of a semiconductor material to be used. When the band gap is small, photons with low energy and a longer wavelength are generated. In order to generate photons with a shorter wavelength, there is required a semiconductor material with a broader band gap.
For example, AlGaInP commonly used in lasers emits light corresponding to visible red light (approximately 600xcx9c700 nm). On the other hand, silicon carbide (SiC) and Group III nitride semiconductor materials such as gallium nitride (GaN) with a comparatively broad band gap emit light corresponding to visible blue light or ultraviolet rays. A short wavelength LED has an advantage in increasing a storage space of an optical storage (approximately 4 times as large as that of a general LED emitting red light).
The same as other Group III nitride semiconductor materials for emitting blue light, there is no practical technique for forming a bulk single crystal made of GaN. Accordingly, there is required a substrate suitable for growing a GaN crystal thereon. Sapphire, i.e., aluminum oxide (Al2O3), is typically used as such a substrate for growing the GaN crystal thereon.
However, a sapphire substrate has an insulating property, thus limiting the structure of a GaN light emitting diode. With reference to FIG. 1, the structure of a conventional GaN light emitting diode is will be described in detail.
FIG. 1 is a cross-sectional view of a conventional GaN light emitting diode 10. The GaN light emitting diode 10 comprises a sapphire substrate 11 and a GaN light emitting structure 15 formed on the sapphire substrate 11.
The GaN light emitting structure 15 includes an n-type GaN clad layer 15a, an active layer 15b formed to have a multi-quantum well structure, and a p-type. GaN clad layer 15c. Here, the n-type GaN clad layer 15a, the active layer 15b and the p-type GaN clad layer 15c are sequentially formed on the sapphire substrate 11. The light emitting structure 15 may be grown on the sapphire substrate 11 using MOCVD (metal-organic chemical vapor deposition), etc. Here, in order to improve the lattice matching of the light emitting structure 15 and the sapphire substrate 11, a buffer layer (not shown) made of AlN/GaN may be formed on the sapphire substrate 11 before the growing of the n-type GaN clad layer 15a. 
The p-type GaN clad layer 15c and the active layer 15b are removed at designated portions by dry etching so as to selectively expose the upper surface of the n-type GaN clad layer 15a. An n-type contact 19 is formed on the exposed upper surface of the n-type GaN clad layer 15a, and a p-type. contact 17 is formed on the upper surface of the p-type GaN clad layer 15c. A designated voltage is applied to the n-type contact 19 and the p-type contact 17. Generally, in order to increase a current injection area while not negatively affecting luminance, a transparent electrode 16 may be formed on the upper surface of the p-type GaN clad layer 15c before forming the p-type contact 17 on the p-type GaN clad layer 15c. 
As described above, since the conventional GaN light emitting diode 10 uses the insulating sapphire substrate 11, the two contacts 17 and 19 are formed on the sapphire substrate so that the contacts 17 and 19 are nearly horizontal with each other. Accordingly, when a voltage is applied to the conventional GaN light emitting diode 10, a current flows over a narrow area from the n-type contact 19 to the p-type contact 17 via the active layer 15b in a horizontal direction. Since a forward voltage (Vf) of the light emitting diode 10 is increased due to this narrow current flow, the current efficiency of the light emitting diode 10 is lowered and an electrostatic discharge effect is weak.
Further, the conventional GaN light emitting diode 10 emits a great amount of heat in proportion to the increase of the current density. On the other hand, the sapphire substrate 11 has low thermal conductivity, thus not rapidly dissipating heat. Accordingly, mechanical stress is exerted between the sapphire substrate 11 and the GaN light emitting structure 15 due to the increased temperature, thus causing the GaN light emitting diode 10 to be unstable.
Moreover, in order to form the n-type contact 19, a portion of the active layer 15b with a size at least larger than that of the contact 19 to be formed must be removed. Accordingly, a tight emitting area is reduced, and the luminous efficiency according to the luminance relative to the size of the diode 10 is lowered.
In order to solve this problem, there is required a vertical light emitting diode. A method for manufacturing the vertical light emitting diode must comprise a step of removing a sapphire substrate from a GaN light emitting structure so as to form a contact layer on upper and lower surfaces of the vertical light emitting diode.
The sapphire substrate may be removed from the GaN light emitting structure using several conventional techniques. Since the sapphire substrate has a high strength, there is a limit to the ability to remove the sapphire substrate from the GaN light emitting structure using mechanical polishing. Further, the removal of the sapphire substrate from the GaN light emitting structure using a laser beam may cause damage to the GaN single crystal plane of the GaN light emitting structure due to the lattice mismatching and the difference of thermal coefficient of expansion (TCE) between the sapphire substrate and the light emitting structure during exposure to the laser beam.
More specifically, when the laser beam is irradiated on the lower surface of the sapphire substrate in order to remove the sapphire substrate from a GaN single crystalline layer, residual stress occurs due to the difference of thermal coefficient of expansion between the sapphire substrate and the GaN single crystalline layer, and the lattice mismatching thereof. That is, the thermal coefficient of expansion of sapphire is approximately 7.5xc3x9710xe2x88x926/K, while the thermal coefficient of expansion of GaN single crystal is approximately 5.9xc3x9710xe2x88x926/K. In this case, the rate of the lattice mismatching is approximately 16%. In case that a GAN/AlN buffer layer is formed on the sapphire substrate prior to the growing of the GaN single crystalline layer, the rate of the lattice mismatching is several percent (%). Accordingly, when the heat is generated by exposure to the laser beam, large-sized compressive stress is exerted on the surface of the sapphire substrate and large-sized tensile stress is exerted on the surface of the GaN single crystalline layer. Particularly, since the area of the irradiation of the laser beam is narrow (maximally 10 mmxc3x9710 mm), the laser beam is repeatedly irradiated on sectional areas of the sapphire substrate so that the laser beam can be irradiated on the entire surface of the sapphire substrate. Thereby, the level of stress becomes more serious, thus excessively damaging the surface of the GaN single crystalline layer.
As a result, the damaged GaN single crystalline plane drastically reduces the electric characteristics of the finally manufactured GaN light emitting diode.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for manufacturing GaN blue light emitting diodes with improved luminance and reliability obtained by stably removing a sapphire substrate from a GaN light-emitting structure.
In accordance with the present invention, the above, and other objects can be accomplished by the provision of a method for manufacturing GaN light emitting diodes, comprising the steps of: (a) forming a light emitting structure on a sapphire substrate, said light emitting structure including a first conductive GaN clad layer, an active layer and a second conductive GaN clad layer sequentially stacked on the sapphire substrate; (b) dividing the light emitting structure into plural units with a designated size so that the first conductive GaN clad layer of a thickness of at least approximately 100 xc3x85 remains; (c) attaching a conductive substrate to exposed upper surfaces of the unit light emitting structures using a conductive adhesive layer; (d) irradiating a laser beam on a lower surface of the sapphire substrate so that the sapphire substrate is removed from the unit light emitting structures, wherein the residual first conductive GaN clad layer is removed so that the light emitting structure is perfectly divided into the unit light emitting structures with a size the same as that of light emitting diodes to be finally manufactured; (e) forming first and second contacts respectively on the surface of the first conductive clad layer, from which the sapphire substrate is removed, and the exposed surface of the conductive substrate; and (f) cutting the resulting structure along the divided lines of the unit light emitting structures into plural unit light emitting diodes.
In the step (b), the thickness, of the residual first conductive GaN clad layer may be preferably less than approximately 2 xcexcm, and more preferably less than approximately 1 xcexcm.
Further, in order to improve the luminance of light emitted from the upper surface of the diode, a reflective layer made of a conductive material may be formed between the second conductive GaN clad layer and the conductive adhesive layer. Preferably, the reflective layer may be made of a material selected from the group consisting of Au, Ni, Ag, Al and their alloys.
Preferably, the conductive substrate may be made of a material selected from the group consisting of silicon (Si), germanium (Ge), SiC, ZnO, diamond, and GaAs, and the conductive adhesive layer may be made of a material selected from the group consisting of Auxe2x80x94Sn; Sn, In, Auxe2x80x94Ag, Agxe2x80x94In, Agxe2x80x94Ge, Agxe2x80x94Cu and Pbxe2x80x94Sn.
Further, in order to obtain an improved current density distribution, the first conductive GaN clad layer may be a GaN crystalline layer doped with an n-type impurity, and the second conductive GaN clad layer may be a GaN crystalline layer doped with a p-type impurity.
Moreover, preferably, in the step (c), the conductive adhesive layer may be formed on the lower surface of the conductive substrate in advance, and then the lower surface of the conductive substrate provided with the conductive adhesive layer may be attached to the exposed upper surfaces of the unit light emitting structures. Alternatively, in the step (c), the conductive adhesive layer may be formed on the upper surfaces of the unit light emitting structures, and then the conductive substrate may be attached to the upper surfaces of the unit light emitting structures provided with the conductive adhesive layer.
In the method for manufacturing vertical GaN Light emitting diodes of the present invention, the GaN single crystalline light emitting structure is grown on the sapphire substrate and the conductive substrate such as a silicon substrate is attached to the other surface of the light emitting structure using the conductive adhesive layer. Subsequently, the sapphire substrate is removed from the light emitting structure using the laser beam. Accordingly, it is possible to more easily manufacture the vertical GaN Light emitting diodes.
Further, in the method for manufacturing vertical GaN Light emitting diodes of the present invention, the light emitting structure is divided into plural units with a designated size so that the first conductive GaN clad layer of a thickness of at least, approximately 100 xc3x85 remains on the sapphire substrate. Accordingly, it is possible to prevent the laser beam passing through the sapphire substrate from reaching and melting the conductive adhesive layer, during the step of removing the sapphire substrate from the light emitting structure using the laser beam.
Here, the residual first conductive GaN clad layer may be removed by mechanical impact indispensably generated when the laser beam is irradiated on the sapphire substrate so as to remove sapphire substrate from the light emitting structure. The thickness of the residual first conductive GaN clad layer may be preferably less than approximately 2 xcexcm, and more preferably less than approximately 1 xcexcm, so that the residual first conductive GaN clad layer can be removed by small mechanical impact.